Tourope Posted July 28, 2017 Share Posted July 28, 2017 I've read several times that you shouldn't leave your mains lead coiled when plugged in. Why is that and what's the science behind it? Only I've left mine coiled on many occasions and in 30C + temperatures and I've never had any problems with it. Link to comment Share on other sites More sharing options...
Keithl Posted July 28, 2017 Share Posted July 28, 2017 Heat build up! And it is not normally a problem in hot weather as your electric usage will be low, it will be a problem in the winter when your usage is high! When in heavy use the cable will warm up and if this heat cannot dissipate it will eventually lead to the cable melting and fusing together, possibly even short circuiting and catching fire! If you have excess cable you should ALWAYS unroll it and lay it out flat on the ground. Keith. Link to comment Share on other sites More sharing options...
mgnbuk Posted July 28, 2017 Share Posted July 28, 2017 Keithl - 2017-07-28 2:17 PM Heat build up! And it is not normally a problem in hot weather as your electric usage will be low, it will be a problem in the winter when your usage is high! When in heavy use the cable will warm up and if this heat cannot dissipate it will eventually lead to the cable melting and fusing together, possibly even short circuiting and catching fire! If you have excess cable you should ALWAYS unroll it and lay it out flat on the ground. Keith. While there may be slight risk of the above if a long 1.5mm2 cable is used with the majority still on a tight reel for long periods with a high load, the above comment seems rather OTT. With a "normal" 20 metre-ish 2.5mm2 cable on a larger diameter reel, unwound to the extent frequently required on most sites & not supplying a continuous 3 KW load, I doubt there will be much in the way of cable heating. But it is as well to be aware of the possibility of a cable overheat risk (however slight) & check to make sure all is OK. Nigel B Link to comment Share on other sites More sharing options...
flyboyprowler Posted July 28, 2017 Share Posted July 28, 2017 Agree, heat! I was on a campsite recently and had my coiled cable 25 metres plugged into the van. The German next door got quite upset and told me that in the past he had witnessed fires from coiled cables. Also if you wind a wire round a nail and supply a power to it, you get a magnet! Better to be safe than sorry, and buy one of the cheap wind up cable drums and uncoil it all every time. Link to comment Share on other sites More sharing options...
PJay Posted July 28, 2017 Share Posted July 28, 2017 flyboyprowler - 2017-07-28 3:40 PM Agree, heat! I was on a campsite recently and had my coiled cable 25 metres plugged into the van. The German next door got quite upset and told me that in the past he had witnessed fires from coiled cables. Also if you wind a wire round a nail and supply a power to it, you get a magnet! Better to be safe than sorry, and buy one of the cheap wind up cable drums and uncoil it all every time. I have noticed , many continentals using a much thinner cable than we in Uk use. Maybe it is only 2 core mot 3 ? maybe thats why it over heats. We always lay out cable out flat when in use PJay Link to comment Share on other sites More sharing options...
BruceM Posted July 28, 2017 Share Posted July 28, 2017 I’ve experienced this in the home environment. I had a 10m extension cable on a reel for home use which I just unwound slightly so that I could get a 2KW fan heater into a room I needed to dry out. After about 45 mins the mains trip switch went. It turned out that the cable insulation had melted on the inner windings. I think that the fan heater would have been pulling about 8 amps so far more than most people’s EHU’s but worth note nonetheless. I now unwind all the way always. Fortunately my EHU cable is not on a reel so does not present the same degree of hazard. Link to comment Share on other sites More sharing options...
Keithl Posted July 28, 2017 Share Posted July 28, 2017 OK I have just done a bit of basic maths and using the approx resistance of 2.5 mm^2 cable have arrived at the conclusion that 2.5 mm^2 cable will dissipate approx 2 Watts per metre when carrying 16 Amps. If you have let's say 10 metres coiled that will be 20 watts of heat generated in a confined space. Imagine holding your hands close to a 20 Watt light bulb for any length of time and you get the idea of the result! Now if your extension cable had only been made of 1.5 mm^2 cable the resistance would be considerably higher and you would now be dissipating approx 3.4 Watts per metre, not far off double the heat to dissipate with even more disastrous results! Does this help explain the problem? Keith. Link to comment Share on other sites More sharing options...
Tourope Posted July 28, 2017 Author Share Posted July 28, 2017 OK, thanks to all for the comments. I thought it might be heat build up but as I say I've never experienced it. I was also aware that a cable in a coil creates a magnet. From what you've said it would seem any electrical cable will give off heat when it is powering something so keeping it coiled the outer coils insulate the heat from the inner coils preventing the inner coils to overheat and eventually melt. My cable is wrapped around a flat spindle, a bit like those you sometimes see for fishing (or weaving),and there will usually be an air gap in the centre. So maybe that's why it's not overheating. Link to comment Share on other sites More sharing options...
tonyishuk Posted July 28, 2017 Share Posted July 28, 2017 Slightly o/t but it always pays to feel how hot the plug and "socket" ends are during use. Any heat build up probably means loose ot chaffed wires. Rgds Link to comment Share on other sites More sharing options...
PJay Posted July 28, 2017 Share Posted July 28, 2017 Keithl - 2017-07-28 5:28 PM OK I have just done a bit of basic maths and using the approx resistance of 2.5 mm^2 cable have arrived at the conclusion that 2.5 mm^2 cable will dissipate approx 2 Watts per metre when carrying 16 Amps. If you have let's say 10 metres coiled that will be 20 watts of heat generated in a confined space. Imagine holding your hands close to a 20 Watt light bulb for any length of time and you get the idea of the result! Now if your extension cable had only been made of 1.5 mm^2 cable the resistance would be considerably higher and you would now be dissipating approx 3.4 Watts per metre, not far off double the heat to dissipate with even more disastrous results! Does this help explain the problem? Keith. So Keith (I am not a physicist or electrician, just a very retired accountant!) In plain english, does that mean that i am right in thinking that any fires of cables, may have been because they where using 2 core? I have seen many use a very large coil of thin black cable on sites. I shall make sure we do not park up next to anny in future!! Pauline Link to comment Share on other sites More sharing options...
Keithl Posted July 28, 2017 Share Posted July 28, 2017 PJay - 2017-07-28 6:36 PM So Keith (I am not a physicist or electrician, just a very retired accountant!) In plain english, does that mean that i am right in thinking that any fires of cables, may have been because they where using 2 core? I have seen many use a very large coil of thin black cable on sites. I shall make sure we do not park up next to any in future!! Pauline No, not 2 core but too thin a conductor size. The regulations say that EHU cables must be of 2.5 mm^2 cable. This will be moulded into the outer casing of the cable if you want to check yours. 'Cheap' cables (or possibly long cables to keep the weight down) will only be 1.5 mm^2 cable and hence not able to handle the higher current satisfactorily. Try looking at various extension cables around your house to see the difference. Keith. Link to comment Share on other sites More sharing options...
spospe Posted July 28, 2017 Share Posted July 28, 2017 I have had a fire in a Scout Hut, caused by an extension lead that was not fully unwound and left running a 2 kw electric fire overnight. A 25 metre lead for a motorhome which has been left coiled up and lying on the ground can get quite warm, but unlikely to cause a fire. The Scout Hut was caused by the extension lead being coiled up inside a cable reel and unable to dissipate the heat sufficiently. Best practice is to fully unwind the lead, then you are safe. Link to comment Share on other sites More sharing options...
Dave R1664232361 Posted July 29, 2017 Share Posted July 29, 2017 It is the same principle as that involved with a 1KW electric fire - remove the wire from the former and it will not glow red. An AC current flowing through a conductor, i.e. the cable, will induce a current into an adjacent conductor (same wire but adjacent on the coil) via the alternating magnetic field (generator principle with a coil rotating within a magnetic field). This current opposes the main current flowing thus effectively increasing the resistance - Resistance = heat. By uncoiling the cable the inductance of these opposing eddy currents cannot happen. Link to comment Share on other sites More sharing options...
Charles Posted July 29, 2017 Share Posted July 29, 2017 There's no magnetic field involved. The current in in the neutral wire is equal and opposite to the live thus cancelling out any effect. If you don't believe me put a clamp meter round a mains lead, it will read zero. The cables get hot because they have a small resistance, therefore they have a voltage drop... times the voltage drop by the amperage being used and you get a wattage. That wattage is what the extension lead itself is consuming. The heat needs to dissipate from the cable, simple as that. Link to comment Share on other sites More sharing options...
Brian Kirby Posted July 29, 2017 Share Posted July 29, 2017 This is not a universally accepted theory. What seems to be the more widely accepted theory is that when electricity flows in a cable it created some heat due to the resistance of the cable. This is, indeed, illustrated by the 1kW electric fire, where a fine wire is used in the element precisely because it will be caused to glow red due to its resistance. The closer the flowing current comes to the rated capacity of the cable, the more heat is generated. If uncoiled this heat dissipates harmlessly, as it is designed to do, into the surrounding air. However, where the cable is coiled the heat generated cannot dissipate to air, so warms the cable. The closer the cable is operating to its rated capacity the hotter it becomes until the insulating softens and the conductors begin to move toward each other until they touch, causing a short circuit and resulting surge in current and heat, which can set the insulation alight. OTOH, a cable that is operating well below its rated capacity will not suffer this fate, as its resistance will not generate sufficient heat. Whether leaving the cable coiled will be a fire risk requires a clear understanding of the maximum connected load, and the rating of the cable. If neither are known, it is best to uncoil. If both are known and the safe limit understood, the cable can be left coiled without risk. Some cable carriers have two load limits written onto them, one for use when uncoiled, the other for use when coiled. Link to comment Share on other sites More sharing options...
Charles Posted July 29, 2017 Share Posted July 29, 2017 "OTOH, a cable that is operating well below its rated capacity will not suffer this fate, as its resistance will not generate sufficient heat" Exactly. Thicker cable = less voltage drop = less wattage consumed by the cable = less heat. Link to comment Share on other sites More sharing options...
Gram Posted July 31, 2017 Share Posted July 31, 2017 Dave 'R' - 2017-07-29 6:46 PM It is the same principle as that involved with a 1KW electric fire - remove the wire from the former and it will not glow red. . Do you know this to be true, Dave? G Link to comment Share on other sites More sharing options...
Rayjsj Posted August 1, 2017 Share Posted August 1, 2017 flyboyprowler - 2017-07-28 3:40 PM Agree, heat! I was on a campsite recently and had my coiled cable 25 metres plugged into the van. The German next door got quite upset and told me that in the past he had witnessed fires from coiled cables. Also if you wind a wire round a nail and supply a power to it, you get a magnet! Better to be safe than sorry, and buy one of the cheap wind up cable drums and uncoil it all every time. I dislike the cheap enclosed orange plastic cable drums, they encourage overheating. I much prefer the open cored oblong heavy duty H- frame type of cable storage cages, they allow the accumulated heat to dissipate before it gets bad enough to damage the cable, even with very little of the 25 metres unwound. And yes i have tested it with 16 amps or close to it flowing. Link to comment Share on other sites More sharing options...
AndyLou Posted August 1, 2017 Share Posted August 1, 2017 I always work under the assumption that If I plan to use the heater or electric hob then I must unwind the cable. If I am just charging the battery and running the fridge then there is no need to unwind it. These discussions seem to suggest that I have made a reasonable assumption. Andy Link to comment Share on other sites More sharing options...
Dave R1664232361 Posted August 1, 2017 Share Posted August 1, 2017 Gram, It's my job. Link to comment Share on other sites More sharing options...
ham Posted August 1, 2017 Share Posted August 1, 2017 Dave 'R' - 2017-07-29 6:46 PM It is the same principle as that involved with a 1KW electric fire - remove the wire from the former and it will not glow red. An AC current flowing through a conductor, i.e. the cable, will induce a current into an adjacent conductor (same wire but adjacent on the coil) via the alternating magnetic field (generator principle with a coil rotating within a magnetic field). This current opposes the main current flowing thus effectively increasing the resistance - Resistance = heat. By uncoiling the cable the inductance of these opposing eddy currents cannot happen. That's it Dave it also can act as a capacitor inducing radio interference . They designed the round cable holder so has not to damage the cable when it is wound on the reel ,the H style will eventually damage the cable, Your bending the cable to fit the holder, same principle as if you keep hold of a wire and bend over and over, the conductor will break down. I have seen this in the cabling industry when an extension cable has been wound around a block of wood. it was easy money for us just to place Link to comment Share on other sites More sharing options...
Tony Jones Posted August 1, 2017 Share Posted August 1, 2017 Interesting thread this. It does make me wonder about the LIKELIHOOD of any damage or danger (what professionals would call "risk assessment.") Having travelled in France for years, and now living here, I've seen scores of mains cables thinner than mine, only partly uncoiled (or conversely uncoiled at great distances, across several pitches, paths or roadways!). And never yet witnessed a fire. Have I (and my "aire-neighbours") just been lucky, or is the actual (mathematical) risk lower than we Brits think it is? Link to comment Share on other sites More sharing options...
Derek Uzzell Posted August 2, 2017 Share Posted August 2, 2017 If a product like this https://www.leroymerlin.fr/v3/p/produits/enrouleur-de-cable-electrique-bricolage-l-50-m-lexman-e143033 has just a couple of metres of cable unwound and a 3kW electric fire plugged into it and run for an extended period, it should be anticipated that the cable that remains coiled on the reel will heat up to the point when fire may occur. If the power demand is much less - just running a motorhome’s battery charger, say, and/or powering a 230V TV - the risk of fire becomes much less. A fire risk warning is given here in the paragraph headed “Cable Safety”: https://www.caravanclub.co.uk/advice-and-training/technical-advice/mains-electrical-installation/ and an instance of cable-damage occurring is mentioned by ‘lohr500’ here https://www.practicalcaravan.com/forum/technical/35990-eletric-hook-up-cable I’ve had a domestic cassetted extension cable overheat due to me not unwinding it fully. The appliance I was using stopped working. The fuses were OK and I even dismantled the cable-reel cassette before realising that the overheating protection function had been triggered. As the appliance’s power demand had not been huge and I had not been using it for that long a period, I had not considered that heat build-up would be significant, but it obviously was. Just because you have not seen a cable-related fire does not mean such fires have not occurred, nor that cables have not become damaged through heat build-up even though they have not burst into flame. Link to comment Share on other sites More sharing options...
hallii Posted August 2, 2017 Share Posted August 2, 2017 Several have touched on conductor size. I often see quite lightweight cables in use. Another factor is voltage, lower voltage increases the current, some of the voltages on continental sites are surprisingly low, 190v on one! H Link to comment Share on other sites More sharing options...
laimeduck Posted August 2, 2017 Share Posted August 2, 2017 Anecdotal evidence:- When skiing in the Alps (using our caravan (not on the slopes - although they do go fast upside down?!)) We attached to the EHU and left the cable coiled under the van in a series of loops (not in a reel). We were using a fan heater to heat the van. The temps went down to -17C and after a few days we noticed that the cable had sunk down through the ice to a depth of about 10cm leaving the impression of the cable in the ice. ie the heat in the cable had melted the ice. It took us several hours using hot water & an axe to free the cable from the ice. Since then we have always uncoiled the cable fully in any temperatures. Jeremy Link to comment Share on other sites More sharing options...
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